Internal wave mixing in warming Lake Grevelingen

TL;DR

This study investigates internal wave-induced turbulence in Lake Grevelingen, revealing episodic mixing processes that influence oxygen levels and thermal stratification during warming periods.

Contribution

It provides detailed measurements of internal wave-driven turbulence in a saltwater lake, highlighting mechanisms that affect oxygen dynamics in stratified coastal lakes.

Findings

Turbulent mixing is episodic and driven by internal waves and wind.

Mean eddy diffusivity is approximately 4x10^-5 m^2/s.

Turbulence is insufficient to prevent summer hypoxia.

Abstract

Seasonal hypoxia or even anoxia can occur in some local deep basins of coastal waters. Such low summertime oxygen contents especially affect benthic life. The seasonal coastal hypoxia is commonly related to biological increased respiration and to physical limited vertical turbulent exchange that is associated with increased vertical stable density stratification. However, the same stratification can support internal waves that may break and locally generate turbulence. Here, we investigate the physics of internal wave motions in saltwater Lake Grevelingen (SW-Netherlands) during warming in mid-spring. Grevelingen is refreshed by weak tidal motions through an open sluice in its dam to the North Sea. The outer North Sea has a surface tidal range of about 3 m, but the lake surface tidal range is negligible (<0.1 m). To quantify vertical turbulent exchange, high-resolution temperature sensors were moored in conjunction with a current meter in 36 m water depth for three days. The site is known for anoxic conditions near the bottom in summer. While a 3-day, 30-m mean eddy diffusivity of <[Kz]> = 4+/-2x10-5 m2 s-1 is found, the overall mean turbulence dissipation rate (turbulent flux) is <[epsilon]> = 1.1+/-0.6x10-7 m2 s-3. Turbulent mixing occurs episodically, via near-surface cooling during night and increased winds, via sparse shear-driven breaking of internal waves at the main pycnocline, and via sheared near-bottom currents. Shear-driven turbulence is not commonly found in fresh-water lakes. Just below the main pycnocline around mid-depth a layer of weak turbulence is observed, as in fresh-water lakes. The observed turbulent exchange is sufficient to warm the near-bottom waters over the course of summer, but insufficient to prevent the biology from over-consuming oxygen contents.